B12 Injections: When They Are Needed and When Tablets Work Just as Well

B12 injections are among the most commonly sold wellness injectables, and the central question about them has actually been answered by good trials. That is unusual in this space and worth taking advantage of.

What the trials found

A Cochrane systematic review compared oral vitamin B12 against intramuscular vitamin B12 for B12 deficiency. A pragmatic randomised non-inferiority trial in BMJ Open ran the same comparison in primary care. A 2025 meta-analysis in Frontiers in Pharmacology extended this to sublingual administration.

The consistent finding is that oral B12 at sufficient dose corrects deficiency comparably to injections in most patients. This is not a fringe result — it is a Cochrane review plus randomised trials plus a meta-analysis pointing the same direction.

When injections genuinely are indicated

The trial findings do not mean injections are never necessary. They are the right choice when:

• Neurological symptoms are present. Rapid, reliable repletion matters, and this is not the place to test absorption.

• Severe deficiency with anaemia requires prompt correction.

• Malabsorption is severe — after gastrectomy or ileal resection, or in significant inflammatory bowel disease.

• Adherence to daily tablets is unreliable, which is a practical reason rather than a physiological one.

• Oral treatment has been tried and levels have not responded.

The comparison

The energy claim

B12 injections are marketed heavily for energy, and this is where the reasoning most often goes wrong.

Correcting a genuine B12 deficiency reliably improves fatigue caused by that deficiency. Giving B12 to someone who is not deficient does not, and there is no mechanism by which it would — excess water-soluble vitamin is excreted.

Who is actually at risk of deficiency

1. Vegans and strict vegetarians — B12 is essentially absent from plant foods.

2. Adults over 60, as gastric acid production and absorption decline.

3. People on metformin long term.

4. People on long-term proton pump inhibitors.

5. Pernicious anaemia — autoimmune loss of intrinsic factor.

6. After bariatric surgery or bowel resection.

The test is inexpensive and widely available. Serum B12, ideally with methylmalonic acid or homocysteine where the result is borderline, establishes whether there is anything to treat. That is a better first step than a course of injections bought on the assumption of deficiency.